Restricted and repetitive behaviors and interests (RRBs) are a defining characteristic of autism. While essential to clinical diagnosis, this group of behaviors is woefully understudied relative to social and communication problems in autism. Specifically, there is limited understanding of the development, course, and uniqueness of RRBs. The objective of this project is to examine RRBs in young children (ages 2 and 3 years of age) with autism or autism spectrum disorder (ASD). RRBs comprise a diverse set of behaviors such as repetitive motor actions, rituals, routines, and compulsions. The presence and quality of RRBs have been shown to adversely affect both adaptive and social functioning and also predict severity of autism symptoms. Characterizing the developmental course of RRBs will allow for examination of their direct impact on social and communication outcomes in autism. This is directly relevant to individualizing treatment as well as constructing subgroups based on clusters of symptoms. Prior investigations of RRBs in autism have studied older individuals or those with severe cognitive impairments, but not young children with autism and a range of cognitive functioning. However, it is clear from studies of young children with developmental disabilities such as Down syndrome that RRBs emerge much earlier and should be investigated in this 2- to 3-year old age period. This study will elucidate the developmental aspects of RRBs in autism versus typical development and developmental disabilities. This comparative, longitudinal study will document the early development of RRBs and how these behaviors differ in three groups of young children: 1) children with autism/ASD between 2 and 3 years of age; 2) children with developmental disabilities (DD) between 2 and 3 years of age, and 3) typically developing children (TD). Both DD and TD groups will be matched to the autism/ASD groups on the basis of mental age. All children will be serially evaluated with a comprehensive battery of measures to assess the occurrence, frequency, and intensity of RRBs as well as social, communicative, and behavioral functioning. This is the first study to examine RRBs in young children with autism or ASD using multiple methods (direct observation and informant report). The current study will (1) document the course and developmental trajectory of RRBs in young children with autism vs. non-autism; (2) examine the relationship of RRBs to social-communicative outcomes in autism; (3) construct groupings of RRBs that may constitute significant subtypes in autism based on co-occurrence, patterns over time, and intensity and (4) use RRBs as the basis for phenotypic stratification in this data set to identify genes associated with RRBs and/or autism.